Monday, August 6, 2012

Mid-level Convection

Last night there was a weak push of marine air into western Washington that will take the edge off the heat today.  Related to my earlier blog, the thermal trough has moved into eastern Washington, easterly flow over the mountains has ceased, and there is an onshore pressure gradient, pushing marine air inland.   This was not a strong marine push and temps will remain at or a bit above normal today.

But something else is going on...a weak upper trough is moving northward through the region, and with relatively unstable air above, we are seeing a line of mid-level convection pass northward across the area.  Here is the radar image around 10 AM... the radar is seeing precipitation, but as we shall see, most of it is not reaching the ground.  Not much lighting..just a single hit in the last hour in eastern WA.

The visible satellite picture shows the line of showers very nicely.  You can also see the low clouds over the Pacific and the fact that some of the clouds started to move inland a bit.


 What is causing this line of clouds and showers?  An upper level trough moving northward--here is the model simulation for 11 AM...the red color is vorticity, a measure of the sharpness of the trough...red is more.
This feature is moving northward and the associated clouds should reach the Canadian border by dinnertime.  Even now, the clearing is approaching Olympia.

Most of the showers are associated with mid-level convection...and much of the limited precipitation is not reaching the ground...evaporated on the way down (see image from UW webcam).  This
morning I noticed so mammatus clouds....hanging hemispherical features extending below some of the cloud base (you can see a suggestion in the image above).  Really neat.  Here is what these kinds of clouds look like:
 So later today you will see clearing (except for the coast) and tomorrow could have some morning clouds that will burn off by lunchtime.   A step cooler and cloudier on Wednesday as the marine influence strengthens.  No heat wave for next weekend....


6 comments:

lhsouthern said...

how long does it usually take for onshore flow to move a thermal low over the cascade mountains?

rainycity1 said...

What's been going on over Spokane these last few days? The radar has consistently shown something, but I don't believe they've had any precipitation. I don't think it looks like the bird swarms that you've shown us previously. Is it a manifestation of another type of radar behavior, or is it something that is really happening?

smokejumper said...

Even though mid level convection has a lot of virga, it still provides unseen moisture and instability to the atmosphere. A sign was relatively high CAPE values.

Very freakish, lone tstorm hit around 2pm. First, convection usually ignites further up the slopes, not lower in the valley. Two, it disappeared as quickly as it formed. Watered the yard, garden, and washed my car for the week.

Then the sun and 100 degree heat cameback. With all the standing water, it was like a sauna. After 8 years of recording weather, my station hit an all time high DP of 72 degrees.

To top it off, an even more instense rainstorm hit this evening a couple miles further west, causing flash flooding down many rock quarry ravines.

richard583 said...

This same system energy moved through Sacramento, were I'd been visiting with my wife, on Saturday - afternoon. A fairly substantial change from the day previous. And with warmer temps filling in certainly in part again by Sunday.

Jim Laite said...

I'm enjoying your segments on KPLU http://www.kplu.org/post/hottest-day-year-so-far-likely-tomorrow?ft=1&f=151531260,152027457,152504688,153012195,153703045,154154387,155103574,155577210,155993927,156375389,156493794,156728273,157104723,157483742,158058206

Thompsonized said...

Awesome Mammatus cloud formation over Ellensburg last night at sunset :)

https://twitter.com/thompsonized/status/232905897553309696/photo/1

https://twitter.com/thompsonized/status/232905953257852929/photo/1

https://twitter.com/thompsonized/status/232905998761857025/photo/1

https://twitter.com/thompsonized/status/232906041610866688/photo/1